Self-assembly of neutral and cationic PdII organometallic molecular rectangles: synthesis, characterization and nitroaromatic sensing

Abstract

Design and synthesis of three novel [2 + 2] self-assembled molecular rectangles 1–3via coordination driven self-assembly of predesigned Pd(II) ligands is reported. 1,8-Diethynylanthracene was assembled with trans-Pd(PEt₃)₂Cl₂ in the presence of CuCl catalyst to yield a neutral rectangle 1via Pd–C bond formation. Complex 1 represents the first example of a neutral molecular rectangle obtained via C–Pd coordination driven self-assembly. A new Pd₂^II organometallic building block with 180° bite-angle 1,4-bis[trans-(ethynyl)Pd(PEt₃)₂(NO₃)]benzene (M²) containing ethynyl functionality was synthesized in reasonable yield by employing Sonagashira coupling reaction. Self-assembly of M² with two organic clip-type donors (L²–L³) afforded [2 + 2] self-assembled molecular rectangles 2 and 3, respectively [L² = 1,8-bis(4-pyridylethynyl)anthracene; L³ = 1,3-bis(3-pyridyl)isophthalamide]. The macrocycles 1–3 were fully characterized by multinuclear NMR and ESI-MS spectroscopic techniques, and in case of 1 the structure was unambiguously determined by single crystal X-ray diffraction analysis. Incorporation of Pd-ethynyl bonds helped to make the assemblies π-electron rich and fluorescent in nature. Complexes 1–2 showed quenching of fluorescence intensity in solution in presence of nitroaromatics, which are the chemical signatures of many commercially available explosives.